avcodec/truemotion2: Check len in tm2_read_stream()

Fixes: Timeout
Fixes: 8774/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION2_fuzzer-5942199639343104

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

libavcodec/truemotion2.c

@@ -377,6 +377,10 @@ static int tm2_read_stream(TM2Context *ctx, const uint8_t *buf, int stream_id, i
             }
         }
     } else {
+        if (len < 0) {
+            ret = AVERROR_INVALIDDATA;
+            goto end;
+        }
         for (i = 0; i < toks; i++) {
             ctx->tokens[stream_id][i] = codes.recode[0];
             if (stream_id <= TM2_MOT && ctx->tokens[stream_id][i] >= TM2_DELTAS) {